Air-coupled capacitive ultrasonic transducers generally have low bandwidths, meaning that a slight variation in the frequency of the power source driving a transducer has a large impact on the magnitude of the mechanical output. The low bandwidths are most prevalent on transducers that have high Q values in that a small change in the frequency of the power source results in a large change in the amplitude of the transducer mechanical output. This large change is greatest around the frequency of the power source that causes the greatest mechanical output. This frequency is sometimes referred to as the peak frequency.
The low bandwidth is present in micro-machined and piezoelectric transducers. The low bandwidth causes poor time domain performance because the transducers become under-dampened, which leads to a slow decay of the ringing after the transducers are excited by their power sources. In addition, the frequency domain performance suffers because the low bandwidth makes use of frequency modulated excitation, such as chirps, from the power sources, less effective. The use of coding is also limited because the low bandwidth requires the use of very long symbols in order to obtain frequency shaping. Therefore, frequency shaping is not attainable with the low bandwidths.
System feasibility is also reduced. The low bandwidths of the transducers adversely affect performance by causing mismatches between adjacent transducers. More specifically, a first transducer may have a peak frequency at f1 and a second transducer may have a peak frequency at f2 that is very close to the frequency f1. Because the bandwidths of the transducers are very narrow, the transducer operating at the frequency f1 may not be able to be matched to the transducer operating at the frequency f2.